When a gas or vapour at a higher pressure passes through an impedance to the ow into a region of lower pressure, then in general a beam is formed. Whether the beam is composed mainly of atoms or of molecules, the beam-forming process is the same, and without loss of generality; we will mainly use the term ‘atomic beam’ to mean either an atomic or a molecular beam. We justify this preference since atomic beams are much more common than molecular beams. However, we note that many authors use ‘molecular beam’ to cover atomic beams as well and ‘particle’ is also used. It is also more usual to refer to gas ow through a bounded space such as a tube as ‘molecular ow,’ and ‘molecular beam epitaxy’ (MBE) is a well-established expression.